Multiple Host-Guest Interactions in Metal-Organic Frameworks Constructed by Inverted Calix[4]arenes

Calixarenes are versatile macrocyclic compounds featuring unique basket-like cavities that are capable of encapsulating guest molecules selectively. Yet, their application potentials as the building blocks for supramolecular assemblies have not been thoroughly explored. In this work, a carboxyl-modified azocalix[4]arene (CAC4A) is selected as the organic ligand to construct metal-organic frameworks (MOFs), an emerging class of porous materials based on coordination units. Herein, three calixarene-based MOFs are developed based on varied metals, including La³⁺, Ca²⁺, and Mn²⁺. Benefiting from the low symmetry of the calixarene ligands, the three MOFs feature abundant structural diversity, wherein two inverted calixarenes are bridged by metal nodes to produce nanocavities. These cavities, or pores, are interconnected to generate one-dimensional (1D) chains or two-dimensional (2D) sheets that are further assembled into porous frameworks. Such a bottom-up assembly not only presents an approach to constructing hierarchical porous structures, but also gives rise to enhanced adsorption abilities based on host-guest interactions. Single crystal X-ray diffraction can also be employed to determine the interactions between the guest molecule, like iodine, and the frameworks directly.